This invention relates to pulsed neutron well logging and more particularly to means for controlling the neutron output of a neutron generator tube used in pulsed neutron well logging.
In recent years pulsed neutron well logging has become a commercially important well logging technique. Pulsed neutron techniques have been utilized for measuring the thermal neutron lifetime or thermal neutron decay time of earth formations in the vicinity of a well borehole, for making activation analyses of elemental constituents of the earth formations in the vicinity of the well borehole, for making porosity measurements of the earth formations in the vicinity of the well borehole and for making inelastic neutron scattering measurements for fast neutrons. In each of these well logging techniques the pulsed neutron source used to generate neutron pulses for the physical measurements has typically been an evacuated tube, deuterium-tritium accelerator type source. Other techniques such as those disclosed in U.S. Pat. No. 3,940,611 call for waveforms other than square wave pulses to be produced by a neutron generator tube. The system of the present invention is capable of providing such other waveforms as may be desired.
Such sealed off or evacuated tube neutron sources generally comprises an outer envelope of glass, metal or some other vacuum encapsulation material, such as ceramic, which houses therein the elements of the neutron generator tube. The elements generally comprise a target which is electrically insulated at a high voltage potential, a source of ions which may be accelerated onto the target by its high voltage potential and a pressure regulator or replenisher element which may be used to stabilize or control the amount of pressure of gas within the evacuated outer envelope. Gas pressures of about 10.sup.-2 mm Hg. are typical for the operation of these tubes.
The replenisher or pressure regulator of neutron generator tubes generally comprises a heater element which is surrounded by a surface which is capable of absorbing or emitting gas molecules of the gas filling the evacuated tube envelope as a function of its temperature. The capability of such a surface for emitting or absorbing gases in the tube envelope is controlled by the temperature of a heating element associated with it. When the heating element is elevated in temperature, the surrounding gas impregnated surface is encouraged to dispel absorbed gases by thermal emission. When the heating element is cooled, the surrounding surfaces associated with it are encouraged to absorb gases from the atmosphere inside the evacuated tube envelope. The amount of gas present in the tube envelope controls the amount of gas present in the ion source and hence, the capability of the ion source to produce positively charged ions of gas for acceleration onto the target material.
In a typical neutron generator tube operation, the gas present in the evacuated envelope may be either deuterium gas or a mixture of deuterium and tritium gas. The target material is impregnated with tritium. Thus when deuterium ions are formed in the ion source and accelerated onto the target by its high voltage potential, the electrostatic Coulomb repulsion between the ions being accelerated and the nuclei of the tritium atoms is overcome and nuclear fusion takes place. This produces the unstable isotope helium 5 which immediately decays by the emission of an approximately 14 MEV monoenergetic neutron characteristic of this decay.
A problem which has been associated with the use of such neutron generator tubes in well logging has been that the output of the neutron generator falls off as a function of time as the tritium in the target material is effectively used by the nuclear reactions and by heating of the target. Also high voltage power supply voltage variations, replenisher current variations and ion source emission capability can cause neutron output to vary.
For most well logging operations it is highly desirable that during a given logging run the average neutron output of the tube remain constant and also as high as possible. High output is desirable to promote the nuclear interactions sought to be measured by the well logging technique in use. Consistency of the neutron output is desirable to promote measurement consistency and to avoid systematic errors.